Solid state starters/controllers have found widespread use for controlling application of power to an AC induction motor. The conventional starter/controller, referred to hereinafter as simply a starter or a controller, uses solid state switches for controlling application of AC line voltage to the motor. The switches may be thyristors such as silicon controlled rectifiers (SCRs) or triacs.
One application for a motor controller is as an elevator starter. The elevator starter may be used to drive a pump motor for an hydraulic elevator. Each time movement of an elevator car is commanded, the starter must start the motor until it reaches operating speed and then operate in a run mode. Such a starter may only be used for the up direction as gravity may be used for the down direction. One type of elevator starter, referred to as a soft starter, changes the on time of the solid state switches to control voltage and to ramp up motor current with a fixed connection.
A motor may be rated for a limited number of starts in a select time interval. A typical maximum number of starts for an hydraulic elevator system is eighty starts per hour. This averages one start every forty-five seconds. The motor may be periodically started to briefly level the car as needed. If leaks or other problems are present, the motor may start several times at each landing to keep the car level at the floor. Furthermore, many systems use mechanical relays in the circuit controlling the motor starter. Problems associated with relays, such as dirty or worn contacts, can cause an intermittent output instead of a continuous output from the relay. Often the problems may exist for long period of time before being detected. The problems themselves can be intermittent and are difficult to diagnose with out witnessing the condition.
Known soft starters may include fault indication. However, in the situation described above, an overload fault is not a clear indicator of what is causing the fault. In addition, a common industry standard is to set the overload at 140% of the applied motor's rated current as under a fully loaded condition the currents can reach these levels. Depending on times between starts, at this setting, the motor may be damaged before the overload trips out.
The motors in hydraulic elevator systems are usually not rated for continuous duty cycle and can be damaged if allowed to run for long periods of time. Hydraulic elevators are typically not used on rises greater than seventy feet. A slow freight car would be able to travel this distance in less than two minutes.
On hydraulic elevator systems, it is important to keep the temperature within a normal operating range for predictable floor leveling. If the temperature of the oil is out of this range, the car may not stop in the desired position. One method is to simply run the pump and motor to recirculate oil from the tank, through the valve and back into the tank. When the oil is recirculating, the currents typically are below the rated current of the motor.
Both mechanical and solid state motor starters typically have overload protection either built into the starter, or as supplemental devices. The tripping time of the overload is a function of the current. This typically is either an (I t) or an (I2 t) function. To those knowledgeable in the function of overload relays it is obvious that the currents must exceed the full load amp setting by some amount (typically 10 to 20%) for an extended time period to cause the overload relay function to trip. If the motor is allowed to continue to run due to a welded contact in the control circuit or a problem in the oil heating circuit, the motor may run until it fails. Often the oil will heat to a temperature which could cause damage to the valve. In extreme cases the motor may catch fire or the oil may give off quantities of smoke. Because the currents are below the overload setting, the overload will not trip.
The present invention is directed to solving one or more of the problems discussed above, in a novel and simple manner.